The present invention relates to a vertical resonator laser diode and a method for fabricating it. The invention thus relates to a vertical resonator laser diode (VCSEL) as has been described for example in Published, Non-Prosecuted German Patent Application DE 198 13 727 A1, corresponding to U.S. Pat. No. 6,317,446. In the VCSEL, an active layer sequence serving for the generation of laser radiation is disposed between a first Bragg reflector layer sequence and a second Bragg reflector layer sequence, each of which has a plurality of mirror pairs. The two Bragg reflector layer sequences form a laser resonator and are disposed together with the active layer sequence between a first and a second electrical connecting contact layer. One of the two Bragg reflector layer sequences is partially transmissive for the laser radiation generated in the active layer sequence, while the other of the two Bragg reflector layer sequences is highly reflective for the laser radiation generated in the active layer sequence.
Such vertical resonator laser diodes are increasingly of interest in the application in optical communications and data technology and also for signal systems or the like. The configuration described in Published, Non-Prosecuted German Patent Application DE 198 13 727 A1 has connecting contacts situated on opposite sides of the component. In the exemplary embodiment described therein, the anode contact layer is situated on the semiconductor surface on the light exit side, while the cathode connection is connected to the n-doped substrate. However, this construction restricts the usability of the VCSEL to an excessively great extent. For specific mounting techniques, such as flip-chip bonding or mounting on simple and inexpensive lead frames, a VCSEL configuration would be desirable in which the electrical connections for the anode and the cathode are situated on a common main surface of the component, for example on the top side of the chip.
It is accordingly an object of the invention to provide in a vertical resonator laser diode containing coplanar electrical connecting contacts which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which enable a construction in which the electrical connecting contacts are disposed essentially in coplanar fashion on one and the same main surface of the component.
With the foregoing and other objects in view there is provided, in accordance with the invention, a vertical resonator laser diode. The laser diode contains a main surface, and two Bragg reflector sequences including a first Bragg reflector layer sequence having a plurality of mirror pairs and a second Bragg reflector layer sequence having a plurality of mirror pairs and defining part of the main surface. The two Bragg reflector layer sequences form a laser resonator. An active layer sequence is provided and has a pn junction and serves for generating laser radiation. The active layer sequence is disposed between the first Bragg reflector layer sequence and the second Bragg reflector layer sequence. A contact-making zone having a relatively high electrical conductivity and a surface defining part of the main surface, is provided. The contact-making zone extends from the main surface at least as far as the pn junction for making a conductive connection with the first Bragg reflector layer sequence. Two mutually insulated, electrical connection contacts are applied to the main surface. The two Bragg reflector layer sequences and the active layer sequence are disposed in a current path between the two mutually insulated, electrical connecting contacts. The two mutually insulated, electrical connection contacts include a first electrical connecting contact connected to the second Bragg reflector layer sequence, and a second electrical connecting contact disposed on the contact-making zone and coupled to the first Bragg reflector layer sequence though the contact-making zone. An insulation layer is provided for electrically insulating the contact-making zone from the second Bragg reflector layer sequence and from the active layer sequence.
With the foregoing and other objects in view there is further provided, in accordance with the invention, a method for fabricating a vertical resonator laser diode. The method includes the steps of:
a) providing a semiconductor substrate;
b) applying a first Bragg reflector layer sequence to the semiconductor substrate;
c) applying an active layer sequence having a pn junction to the first Bragg reflector layer sequence;
d) applying a second Bragg reflector layer sequence to the active layer sequence resulting in a formation of a laser resonator being formed by the first Bragg reflector layer sequence, the active layer sequence and the second Bragg reflector layer sequence;
e) forming a contact-making zone of relatively high electrical conductivity extending from a surface of the second Bragg reflector layer sequence at least as far as the pn junction;
f) forming an insulation zone serving for electrical insulation between the contact-making zone and the second Bragg reflector layer sequence and the active layer sequence; and
g) applying electrical connecting contacts to a common main surface of the vertical resonator laser diode such that a first connecting contact is connected to the second Bragg reflector layer sequence and a second connecting contact is connected to the contact-making zone.
In this case, method steps e) and f) can be executed in the order specified above, but also, in principle, in the opposite order. In the latter case, first the insulation zone is produced between the second Bragg reflector layer sequence and a section provided as the contact-making zone, and then the contact-making zone is formed, in accordance with method step e), in the section provided therefor.
The insulation zone between the contact-making zone and the second Bragg reflector layer sequence can be fabricated, for example, by shaping a trench between the contact-making zone and one Bragg reflector layer sequence by vertical patterning, that is to say essentially by one or more etching steps, and filling the trench with an electrically insulating material.
In a preferred exemplary embodiment, however, not only is the trench shaped, but larger regions of the second Bragg reflector layer sequence and of the active layer sequence are removed by vertical patterning around a light-emitting region of the laser diode that is to be formed, so that the light-emitting region remains as a mesa-type structure. Before the removed regions are then filled with the electrically insulating material, current aperture layers can then be shaped using the free-standing mesa-type structure in a manner known per se (see Non-Prosecuted German Patent Application DE 198 13 727 A1). In the case of a laser diode based on III-V material, the layers can be fabricated by oxidizing layers with a relatively high aluminum content, as a result of which an oxidized, annular-and peripheral section adjoining the side walls of the mesa-type structure is formed in a manner dependent on the process conditions, the oxidation, the aluminum content and the thickness of the layer. The current aperture layer may be disposed within the first or the second Bragg reflector layer sequence. The width of the annular section, that is to say the diameter of the current conduction region, can also be set in a relatively targeted manner by the process conditions. After the fabrication of the one or the plurality of current aperture layers, the regions removed during the mesa etching can then be filled with the electrically insulating material.
If the intention is to fabricate a vertical resonator laser diode based on III-V material, it is possible, as early as during the growth of the layer structure, to grow layers with such a material composition either as intermediate layers between the mirror pairs or as partial layers of the mirror pairs in the first or the second Bragg reflector layer sequence, so that current aperture layers can be formed from them in a later process. In the case of a vertical resonator laser diode based on (Al, Ga) As, such layers which are provided as current aperture layers are shaped with a relatively high aluminum content.
The contact-making zone can be fabricated by impurity atoms being introduced into the component by diffusion or implantation. In the exemplary embodiment described below, these impurity atoms are introduced on the main surface of the component on the light exit side. They are intended to produce a relatively high electrical conductivity in the contact-making zone. The impurity atoms may thus be, for example, a dopant that produces a relatively high n-type or p-type doping in the second Bragg reflector layer sequence in the region of the contact-making zone to be produced. In the case of III-V material, by way of example, a doping with zinc atoms, that is to say a p-type doping, may be performed. The latter may be performed for example by diffusion at about 600xc2x0 C.-650xc2x0 C. In this case, it is also possible to bring about intermixing of the layers with a different Al content (disordering). This effect does not constitute a disadvantage for the function of the component, but rather provides for a contact-making zone with good electrical conductivity without disturbing heterointerfaces. Before the diffusion step is carried out, masking may be performed by depositing, for example, a masking layer, such as a Si3N4 layer, with a mask opening in the region of the contact-making zone to be shaped on the main surface of the component. The size and shape of the mask opening define the lateral boundary of the contact-making zone, while its depth is determined by the process conditions of the diffusion.
The introduction of impurity atoms with the aim of doping can alternatively also be carried out by an ion implantation. In this case, a mask layer, such as a Si3N4 mask layer, may likewise be used. The implantation can be carried out by a number of implantation steps with suitably chosen implantation energies and doses, so that a contact-making zone is fabricated with an electrical conductivity that is sufficiently homogeneous over its depth.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a vertical resonator laser diode containing coplanar electrical connecting contacts, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.